1. Field of the Invention
The present invention relates to a microwave oven, and more particularly, to a microwave oven waveguide structure which allows the use of low frequency microwaves that are sufficient for cooking without increasing the size of the waveguide.
2. The Description of the Prior Art
FIG. 1 is a view showing an inner housing of a microwave oven having a conventional waveguide, and FIG. 2 is an enlarged perspective view of portion Y in FIG. 1.
The inner housing 10H forms a cavity 10 therein for receiving food and on one side of the inner housing 10H is mounted a waveguide 2 for introducing microwave energy generated by a magnetron (not illustrated) into the cavity 10. A hole 2a for inserting an antenna (not illustrated) of the magnetron on the waveguide 2 is formed on the surface of the waveguide 2 which is opposite the surface that is in contact with the inner housing 10H. A square hole (not illustrated) for sending a microwave to the cavity 10 is formed on the surface (a) of the waveguide 2 placed in contact with one side of the inner housing 10H.
The conventional waveguide 2 is formed as a highly conductive rectangular pipe with a central hole formed on the surface which is opposite the surface that is in contact with the inner housing 10H. The waveguide 2 acts as a high pass filter. The wavelength of the microwaves that pass through the waveguide 2 depend upon the length of the waveguide 2 itself. Wavelengths having a frequency that is greater than a certain frequency do not pass through the waveguide 2. The wavelength is a maximum wavelength of the waveguide 2 which is referred to as a cut-off wavelength, and such certain frequency is referred to as a cut-off frequency.
If a frequency to be used as a magnetron is determined with a magnetron, a minimum length (a) of the waveguide 2 is determined. Here, the relation for obtaining the length (a) of the waveguide 2 is as follows: EQU a.gtoreq..lambda./2 (1)
wherein `a` denotes a length of the waveguide, and .lambda. a wavelength of the used frequency.
As shown in the relation (1), when the wavelength of the used frequency .lambda. is substituted in the relation (1), the minimum length (a) of the waveguide to be used is determined. Thereby, the cross-sectional size of the waveguide according to the cut-off frequency can be determined.
For example, in a domestic microwave oven generally using a frequency of 2,450 MHz, the wavelength (.lambda.) is 122.4 mm, the length (a) of the waveguide 2 is about 62 mm according to the relation (1). Therefore, it can be concluded that when the length of the waveguide 2 is at least 62 mm, the 2,450 MHz of the microwave generally used can be passed.
Moreover, the frequency used in a commercial microwave oven is generally 915 MHz which is relatively low in comparison with that of the domestic microwave oven and has a high infiltration degree of microwaves to food, and especially has a good thawing ability. Since the wavelength (.lambda.) of the frequency is 328 mm, the length (a) of the waveguide 2 is about 164 mm. Therefore, it can be concluded that when the length of the waveguide 2 exceeds at least 164 mm, microwaves of 915 MHz generally used can be passed.
When the rectangular waveguide according to the conventional art is adopted, the domestic microwave having about 100 mm of the length of the waveguide cannot pass the low frequency of 915 MHz. Therefore, only the high frequency of 2,450 MHz must be used. Accordingly, if the domestic microwave oven adopts 915 MHz of the low frequency, the length of the waveguide 2 must be formed to be at least 164 mm, which causes the size of the waveguide, and the volume of the cavity to be increased so as to correspond to that of the waveguide.